Anatomy clinical correlates: Eye

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Introduction0:00–0:35

The eyes enable us to see trees of green, red roses too… and basically everything in the wonderful world around us. From an anatomical perspective, the eyes are sensory organs, and they’re protected by a hard bony casing called the orbit, and shielded from the outside environment by softer tissues like the eyelids.
Unfortunately, all of these structures are prone to various diseases - but luckily, understanding eye anatomy can help us recognize and treat these conditions!

Eyelids0:35–1:37

Let’s start with the eyelids. Remember that they’re controlled by a muscle called the orbicularis oculi, which is a ring of muscles with two different parts that are arranged in circumferential bands around the orbit.
The outer and thicker ring is the orbital part, while the thinner part that lies nearer to the eyelids is known as the palpebral part.
The orbicularis oculi muscles are innervated by the facial nerve, and when they contract, they bring the eyelids together to close the eye for protection.
So with a facial nerve lesion, the function of the orbicularis oculi muscle is affected, which impairs the muscle’s ability to close the eyelids.
First, this means that blinking and moisturizing the front of the eye with lacrimal secretions is impaired, so the cornea can dry out.
Then, there is also the added risk of foreign bodies entering the eye due to impaired blinking, for example sand blowing into our face during a windstorm.
Abrasions and infections can then result which can ultimately lead to corneal ulceration. Now, the eye can also be subject to infection even if the eyelids are working properly.

Infections1:37–4:04

One of the most common ones is hordeolum - usually referred to as a “stye”. This is an abscess of the eyelid, typically presenting as localized erythematous and painful swelling on the eyelid.
A hordeolum can be external, which is when it arises from either the gland of Zeis or the gland of Moll which both secrete sebum in the eyelash follicle on the margin of the eyelid.
A stye can also be internal, in which case it arises from the meibomian gland, causing a swelling under the conjunctival side of the eyelid.
The meibomian glands are the tiny oil glands at the roots of our eyelashes. Another type of eyelid lesion known as a chalazion has an origin similar to that of a stye, a blocked duct of a gland, and it can also even develop from an old stye.
Chalazions, however, differ from the hordeolums as they generally present as a painless swelling on the inner part of the eyelid, and more often than not, they involve the meibomian gland.
Another common eye condition is conjunctivitis, also known as pink eye. Conjunctivitis means inflammation of the conjunctiva, which is a mucous membrane on both the inner surface of the eyelids covering the eyeball, as well as globe of the eye except for the cornea, which is the transparent part of the globe located just over the iris and the pupil.
On the other hand, when the cornea is inflamed, that’s called keratitis. And just to put two and two together, when it's both the conjunctiva and the cornea that are inflamed, that’s called keratoconjunctivitis.
Now let's take a quick look at uveitis, which is an inflammatory condition of the eye that’s commonly associated with systemic inflammatory disorders.
Remember that the uvea is the middle layer of the eye which lies between the outer sclera layer and the inner retinal layer.
This vascular and pigmented layer of the uvea is made up of the choroid, the ciliary body and the iris. So, based on anatomic positions of these structures, anterior uveitis will involve the iris and can sometimes be referred to as iritis; and posterior uveitis will involve the choroid and can sometimes be referred to as choroiditis.
Because of the choroid’s intimate relationship with the retina, posterior uveitis can also involve the retina leading to additional retinitis or chorioretinitis when both layers become inflamed together.Ok now, time for a quick quiz!

Quiz4:04–4:18

Can you recall the parts of the eye that are inflamed with conjunctivitis, keratitis and uveitis?Okay, now let’s switch gears and look at glaucoma.

Glaucoma4:18–6:37

This term refers to a group of eye diseases in which there is damage caused to the optic nerve. Usually, but not always, glaucoma results from an abnormally high intraocular pressure.
Remember that normally, the aqueous humor produced by the ciliary body is drained by the trabecular meshwork found in the angle of the anterior chamber.
So basically, anything disrupting the flow of aqueous humor through this meshwork can cause an accumulation of aqueous humor, which will ultimately result in increased intraocular pressure.
In time, this buildup of pressure against the optic nerve can cause damage to it and lead to vision loss.Now, two of the main types of glaucoma are closed-angle, and open-angle.
Closed-angle glaucoma occurs when the iris bulges forward, for example due to an enlarged lens, and narrows or closes the angle of the anterior chamber, which is the angle formed laterally by the cornea and iris where drainage occurs.
The closure or tightening of this angle impairs the drainage of the aqueous humor, resulting in an increased IOP and subsequent damage to the optic nerve.On the other hand, with open-angle glaucoma, the anterior chamber angle is not decreased.
Instead, open-angle glaucoma occurs as a result of the clogging of the trabecular meshwork, which still leads to reduced drainage of the humor, causing a gradual increase in IOP that once again damages the optic nerve.
Both open and closed glaucoma if left untreated result in progressive and irreversible visual loss. Finally, bear in mind that glaucoma can also be classified as acute or chronic.
Traditionally, open angle glaucoma is the slowly progressing, chronic type, that steadily causes damage to the optic nerve over time.
Acute glaucoma, on the other hand, typically refers to closed-angle glaucoma that has an acute onset, and symptoms may include severe eye pain, blurry vision, redness of the eye, sudden loss of vision, or photophobia.
This is called acute angle-closure glaucoma and should be treated urgently.Ok now, another important condition that involves the eye is Horner syndrome.

Horner syndrome6:37–10:21

So let’s take a closer look at the sympathetic innervation of the eye first! Normally, the sympathetic innervation to the eye consists of a 3-neuron arc, which is a pathway consisting of three orders of neurons.
The first order neuron consists of central sympathetic fibers, which arise from the posterolateral hypothalamus, descending through the midbrain and pons until the first synapse, which is located in the cervical spinal cord, at the C8-T2 level.
The second order neuron consists of preganglionic pupillomotor fibers contained within the sympathetic trunk. On its way up to the head, the second order neuron passes over the lung apex, and past the brachial plexus roots.
Ultimately these second order fibers ascend to the superior cervical ganglion, which is located alongside the internal carotid artery near the angle of the mandible, just above the level of the common carotid artery bifurcation.
Finally, the third order neuron takes over, and it continues as a set of branches that cover the internal carotid artery and pass through the cavernous sinus with it.
From here, the fibers enter the orbit and the eye to provide sympathetic innervation to the dilator pupillae muscle and the superior tarsal muscle, also known as Müller muscle.
This last muscle is a small muscle responsible for elevating and retracting the upper eyelid along with its bigger cousin, the levator palpebrae superioris.
Recall that the levator palpebrae superioris is innervated by the oculomotor nerve, so some upper eyelid lifting is still possible even when Horner’s syndrome is present.Now, Horner syndrome can be caused by a lesion anywhere along the sympathetic pathway that supplies the eye.
When the lesion causing Horner’s syndrome is along the first-order neuron, it’s referred to as first-order Horner’s syndrome.
These lesions include central infarctions, strokes, tumors that can occur in the hypothalamus, brainstem or cervicothoracic spinal cord above T1, and other conditions such as syringomyelia and cervical spinal cord trauma.
Second-order Horner’s syndrome, on the other hand, occurs as a result of a lesion involving the sympathetic trunk commonly at the thoracic outlet, or at the lung apex.
Damage can also be in the neck where the cervical sympathetic ganglia are located, which can occur iatrogenically from surgery in the neck.
Second-order lesions at the lung apex are typically associated with Pancoast tumors, a type of lung cancer affecting the apex of the lungs.
Finally, third-order Horner’s syndrome can be seen in lesions affecting the internal carotid artery, like a dissection; and other vascular lesions such as cavernous sinus aneurysm, or iatrogenically from carotid artery stenting.
Basically, anything that can compress or injure the third-order neuron can cause third-order Horner’s syndrome! Now, Horner syndrome causes a typical triad of symptoms.
First, there is ptosis, which is the drooping of the upper eyelid, due to loss of the sympathetic innervation to Müller's muscle.
Second, there is anhidrosis, which is the absence of sweating caused by impaired sympathetic innervation to facial sweat glands.
Third, there’s miosis, which is constriction of the pupil caused by disruption of the sympathetic innervation of the dilator pupillae muscle, resulting in unopposed parasympathetic innervation.
You can remember these features using the mnemonic PAM plays the French horn!Ok, now let’s switch gears and take a quick look at central retinal artery occlusion.

Central retinal artery occlusion10:21–11:02

That’s when the central retinal artery is blocked, often by an embolus. The central retinal artery arises from the ophthalmic artery, which is a branch of the internal carotid artery.
Central retinal artery occlusion is commonly seen in people with atherosclerosis of the ipsilateral internal carotid artery.
Much of the retina and part of the optic nerve is supplied by the direct terminal branches of the central retinal artery, so obstruction in the central retinal artery will cut off blood supply to these structures, resulting in a sudden, painless, total loss of vision of the affected eye.Alright, as a quick recap… Orbicularis oculi paralysis can occur because of a facial nerve lesion, and it impairs the ability of the eye to close.

Review11:02–12:40

This can cause the cornea to dry out, and leave it exposed to injuries. An external hordeolum, classically known as a “stye” is a painful swelling caused by an inflammation of the glands of Zeis and glands of Moll in the eyelid.
Internal hordeolums are caused by inflammation of the meibomian gland. A chalazion is also caused by the obstruction of Zeis or meibomian glands but it presents without pain.
There’s also conjunctivitis and keratitis, which are inflammatory conditions affecting the conjunctiva and the cornea, respectively.
We also have uveitis, which is an inflammation of any anatomical part of the uvea. With glaucoma, there’s usually, but not always, increased intraocular pressure.
Glaucoma can be classified as open-angle glaucoma and closed-angle glaucoma. Typically, open-angle glaucoma is a chronic condition that progresses over time, whereas closed-angle glaucoma can have an acute onset.
Horner’s syndrome presents as ipsilateral partial ptosis caused by the sympathetic denervation of Müller's muscle; miosis caused by the interruption of sympathetic fibers to the dilator pupillae muscle, and anhidrosis, which occurs as a result of denervation of facial sweat glands.
Finally, central retinal artery occlusion presents as an acute, painless, monocular loss of vision. It commonly occurs as a complication of emboli and/or atherosclerosis